1. Field of the Invention
The invention is in the field of cutters for cutting tapered holes in metal workpieces.
2. Description of the Prior Art
Tapered interference preload fastening of highly stressed structures in accordance with the teachings of Zenzic U.S. Pat. No. 3,034,611 is currently and has for a number of years been widely utilized in aircraft and space vehicles, and in some of the high performance aircraft it is the only type of fastening capable of providing the required strength-to-weight ratio and fatigue characteristics for a number of the structural joints. However, prior to the present invention such tapered interference preload fastening has been considerably more expensive than conventional straight hole fastening, primarily because of the problems involved in reliably preparing tapered holes with the required dimensional accuracy and finish.
The conventional practice for preparing tapered holes in metal structures to receive tapered interference fasteners is to first drill a tapered hole with a tapered twist drill, and then to employ a second, reaming operation utilizing a tapered twist-type reamer to produce the final tapered hole dimensions and finish. This duplication of effort required in adding the reaming step after the drilling step has generally been found necessary where twist-type drills and reamers were employed to achieve the reliability of hole quality necessary to assure reliability of the interference preloading in the structures which are being fastened. In addition to this problem of duplication of effort, the twist-type drills and reamers are characteristically complex and expensive to make, and have a relatively limited operational life. An example of a typical twist-type reamer conventionally employed in the preparation of tapered holes for receiving tapered interference type fasteners is found in Briles U.S. Pat. No. 3,460,410.
Accordingly, with the conventional practice of using twist drills and twist-type reamers to produce tapered holes of the required dimensional reliability and bore finish for receiving tapered interference fasteners, both the tools and the drilling and reaming operations were much more difficult and expensive than for the preparation of straight, cylindrical holes.
Applicants are not aware of any prior art drill or cutter other than those of the twist type mentioned above that is capable of producing a tapered hole in a metal workpiece suitable for receiving a tapered interference preload fastener in a system such as that disclosed in Zenzic U.S. Pat. No. 3,034,611. More particularly, applicants are not aware of any prior art blade-type cutter capable of producing a tapered hole in a metal workpiece suitable for such a tapered interference fastening system.
One of the most difficult problems to cope with in the use of both prior art blade-type cutters and twist-type drills and reamers is a tendency for the point to drift or wander as it travels through the workpiece, due primarily to non-homogeniety of the workpiece material. This problem is compounded in the case of twist drills and reamers because of their tendency to unwind. This is one of the reasons why successive drilling and reaming operations have heretofore generally been required to produce tapered holes of sufficient reliability for use in a high performance tapered interference fastening system.
Another serious problem in the use of prior art cutters, drills and reamers for producing high quality tapered holes is that such prior art devices conventionally produce chip strings which tend to be difficult to eject from the hole, and which have a tendency to gall and thereby spoil the surface finish of the hole.
Johnson U.S. Pat. No. 3,238,836 discloses a tapered cutter blade, but such blade is a self-tapping lead-in part of a sheet metal screw which produces a complex reaming extrusion at the margin of the aperture in the sheet metal for gripping by the threads of the screw. The lead-in blade part of the Johnson screw is not suitable for cutting a tapered hole in a metal workpiece; it has a non-uniform taper, and has a single asymmetrical cutting edge with a positive rake angle. Skelton U.S. Pat. Nos. 1,405,020 and 1,405,021 disclose blade-type tapered reamers which have no points and consequently cannot cut holes, and which employ positive rake angles at their cutting edges; the Skelton reamers also not being suitable for cutting tapered holes in metal workpieces. Blade-type cutters for producing straight holes are disclosed in Southall U.S. Pat. No. 3,920,350, Backus U.S. Pat. No. 82,583 and Warner U.S. Pat. No. 67,825. Prior art non-twist straight hole cutters of this general type are normally only usable for cutting holes in wood, with the fine wood particles being allowed to build up in the hole. Attempts to cut metal with such tools would normally produce uncontrollable metal chip strings.